M.P. Taylor

Diffusion Cells and Chemical Failure of MCrAlY Bond Coats in Thermal-Barrier Coating Systems

It is proposed that bond coats in thermal-barrier coating (TBC) systems, particularly those deposited by plasma spraying, can contain regions which are diffusionally isolated from the bulk of the coating. This can arise through the internal formation of alumina layers as a consequence of the ingress of molecular oxygen into the relatively porous structure. Such isolated regions, termed diffusion cells, will experience enhanced depletion of aluminum as a result of the continued thickening of the alumina layer at their surface. This process has been demonstrated for a CoNiCrAlY bond coat after oxidation in air at 1100°C. A consequence of this enhanced depletion is that chemical failure will occur sooner in diffusion cells and voluminous breakaway oxides will form above them at the interface of the bond coat and the ceramic top coat. The associated spatial variation in oxidation and displacement rates across the surface of the bond coat are expected to aid delamination of the outer ceramic layer.

Creep relaxation and the spallation of oxide layers

Abstract The ability to predict the onset of spallation of protective oxide layers is an important requirement in evaluating the endurance of high temperature coatings or thin-sectioned components. Oxide spallation tends to occur during cooling, when the oxide layer is usually in compression. Considerable progress has been made in modelling spallation from flat substrates under such conditions, and much of this work is reviewed in this paper. In particular, the results of finite-element modelling of oxide/metal interfacial cracking are presented for both chromia-and alumina-forming substrates. It is shown that creep relaxation in the coating or alloy can retard the growth of the interfacial crack even during fast cooling but the extent of retardation increases with lower cooling rates. When such creep relaxation is extensive, i.e. at low cooling rates or with substrates weak in creep, improved spallation resistance results. For such cases, a simple critical strain-energy approach can be used to predict the onset of spallation.

The effect of bond coat oxidation on the microstructure and endurance of a thermal barrier coating system

Abstract Isothermal oxidation tests have been carried out on a thermal barrier coating (TBC) system consisting of a nickel-based superalloy, CoNiCrAlY bond coat applied by HVOF and yttria-stabilised zirconia (YSZ) top coat applied by EB-PVD. Bond coat microstructure, coating cracking and failure were characterised using high resolution scanning electron microscopy complemented with compositional analyses using energy dispersive X-ray spectrometry. A protective alumina layer formed during the deposition of the YSZ top coat and this grew with sub-parabolic kinetics during subsequent isothermal oxidation at temperatures in the range 950 to 1150°C. After short exposures at 1050°C and final cooling, small sub-critical cracks were found to exist within the YSZ but adjacent to bond coat protuberances. Their formation is related to the development of local tensile strains associated with the growth of an alumina layer (TGO) on the non-planar bond coat surface. However, for the specimens examined, these cracks did not propagate, in contrast to other TBC systems, and final spallation was always found to have occurred at the bond coat/TGO interface. This shows that the strain energy within the TGO layer made a significant contribution to the delamination process.

Evidence for the Formation of Al-Rich Reservoir Phases Resulting from Interdiffusion between MCrAlY Coating and Ni-Based Substrate

In this paper the results of a study of the interdiffusion between an LPPS CoNiCrAlY coating with a Ni-based CM186 alloy substrate are shown. Accelerated oxidation testing at 1200°C is used to demonstrate the sequence of changes occurring to the microstructure of the interdiffusion zone with increasing time at temperature. The evolving microstructural features have been identified using high resolution scanning electron microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). Elemental profiles of the diffusion zone at different times at temperature are compared to a computational model predicting such profiles. It is found that aluminium ingress into the alloy, as predicted by the model, is inhibited by the formation of Al-rich phases in the interfacial zone. With extended time at temperature, aluminium depletion within the coating triggers the dissociation of these phases. These then act as reservoirs and supply aluminium back into the coating.

A method for evaluating the creep properties of overlay coatings

Abstract A new method is described for the evaluation of the creep properties of as-deposited overlay coatings. The method uses a composite tensile specimen consisting of a core alloy of well-characterised creep properties onto which the overlay coating is deposited. The overall strain/time response of this specimen, tested under constant stress conditions, can then be deconvoluted to obtain the creep characteristics of the coating. The method has the advantage that the coatings are evaluated for the same microstructural conditions as they will be used under, i.e. having the same porosity levels and microstructure. A limited demonstration of the method has been made for an air plasma-sprayed Ni25Cr6AlY coating deposited onto an austenitic steel core and creep tested at 900°C. The creep strength of this coating was surmised to be higher than that of equivalent monolithic samples produced by low pressure plasma spraying.

The Oxidation and Interdiffusion of a Chromia Forming Multilayered TBC System

Current thermal barrier coating systems (TBCs) rely on an alumina-forming bond coat to provide protection against oxidation. At bond coat temperatures below 900xa0°C, however, optimum protection against oxidation and hot corrosion is best achieved by a chromia layer. TBC systems using this approach are currently being developed and an example is examined in this paper. The multilayered bond coat in the present TBC system consisted of: a NiCrAlY layer aluminised at its surface; a 50:50 NiCr layer (containing 1.8xa0wt% Si); an air plasma sprayed yttria stabilised zirconia top coat. These samples were oxidised in laboratory air at 750, 800 and 850xa0°C for durations up to 1,000xa0h. Post-test characterisation was undertaken using SEMs and energy dispersive X-ray spectroscopy to determine the growth rate of the TGO and to investigate interdiffusion within the layered coating structure. The interdiffusion process was also modelled using the ODIN finite difference code.

An oxidation study of an MCrAlY overlay coating

Abstract Metallic overlay coatings of the MCrAlY type (where M is Ni, Co or a combination of both) are regularly used in high-temperature plant to provide resistance to oxidation and high-temperature corrosion through the formation of a protective surface layer of alumina. Such coatings are also frequently used as a bond coat in thermal barrier coating systems. For both applications, the endurance of the coating system depends on the maintenance of the protective alumina layer. In particular, a high resistance to spallation is required and also a sufficient reservoir of aluminium within the coating so that re-healing of the alumina layer will occur should spallation occur. In this paper the results of a study into the oxidation behaviour of an LPPS MCrAlY coating on a CM186LC alloy are presented. During the study extensive TGO thickness measurements were made at varying times at 1100°C from which the oxidation kinetics have been calculated up to the point where extensive spallation occurred. Over the time range where partial spallation of the TGO occurred, between 50 to 100 h, a duplex structure in the oxide scale was revealed. Micrographs and analyses of the oxide are presented and the two values of TGO thickness at this time at temperature are shown and discussed in terms of the early oxidation kinetics.

Formation of diffusion cells in LPPS MCrAlY coatings

Abstract The formation of diffusionally isolated regions in air plasma-sprayed, APS, MCrAlY bond coat or overlay coatings has been established. The ‘diffusion cells’ experience greatly accelerated depletion of aluminium and an early onset of breakaway oxidation associated with the morphology and extent of the internal oxides. In this paper the effect of internal oxides produced in low pressure plasma sprayed, LPPS, CoNiCrAlY coatings are presented. Isothermal tests in laboratory air at 1200°C were conducted with post-test characterisation performed using scanning electron microscopy and energy dispersive X-ray analysis. A parabolic correlation was found between TGO growth rate and time at temperature. Importantly, compositional profiles of cross sections through the coatings demonstrated that no barriers to diffusion occurred throughout the body of the coating. What has been found in the LPPS coatings is a mechanism whereby diffusion cell formation is possible at sites where splat boundaries containing internal oxides intersect the outer surface. Gas access is possible at these sites and, with increasing time at temperature, formation of a continuous internal oxide layer forms. The sequence of events at these sites follows that of the diffusion cell model presented earlier and a prediction of time to failure is made.

Observations of the spallation modes in an overlay coating and the corresponding thermal barrier coating system

Abstract The oxidation dynamics of an overlay coating and the corresponding thermal barrier coating system are presented. The particular systems examined are composed of a nickel-based superalloy with an air plasma-sprayed NiCrAlY bond coat and the thermal barrier coating system consists of air plasmasprayed yttria stabilized zirconia layer. Failure can occur in these systems by crack propagation within the ceramic outer layer at the interface with the bond coat. Defects, such as microcracks and pores, are common in plasma-sprayed coatings and within the thermally grown oxide scales. These can act as initiation sites for cracks. The subsequent growth of these cracks can lead to loss of the outer protective materials. Considerable information is available by microscopic examination of sections through test specimens that have been held at temperature for varying amounts of time. By careful sample preparation it is possible to monitor the development of the oxide scale formed during high temperature testing and the sites of failure. Identification of the initiation sites and growth of cracks is important in understanding the spallation process. In this study, scanning electron microscopy is used to provide evidence of the processes involved in the two systems. A comparison of the two coating systems reveals the effect the outer ceramic layer has on the oxide scale growth, and the spallation processes crucial to the understanding of the failure mechanisms of these coating systems.

Effect of prior oxidation on high cycle fatigue performance of RR1000 and role of oxidation in fatigue crack initiation

Abstract The effect of prior oxidation on the room temperature fatigue life of coarse grained Ni based superalloy, RR1000 has been performed at an R ratio of 0·1 with two pre-oxidation times: 100 and 2000 h at 700°C. These pre-exposures produce extensive oxidation damage. The room temperature high cycle fatigue life of the pre-oxidised specimens has been compared to as received specimens. At a maximum applied stress of 800 MPa a significant fatigue life deficit is observed in the pre-oxidised testpieces. This is accompanied with the observations of significant cracking of the external chromia scale and the intergranular internal oxides within the area of maximum stress. Preferential cracking of oxides may lead to early crack initiation and consequently a reduction in total fatigue life.